Method for forming identification marks on silicon carbide single crystal substrate, and silicon carbide single crystal substrate

ABSTRACT

A method for forming an identification mark on a silicon carbide single crystal substrate according to the present invention includes: (a) scanning a principal surface of a silicon carbide single crystal substrate with a laser beam at a first energy density such that a groove is formed in the principal surface of the silicon carbide single crystal substrate, thereby forming an identification mark which is constituted of one or more grooves in the principal surface of the silicon carbide single crystal substrate; and (b) scanning an inside of the groove formed in the principal surface of the silicon carbide single crystal substrate with a laser beam at a second energy density that is lower than the first energy density.

TECHNICAL FIELD

The present invention relates to a method for forming an identificationmark on a silicon carbide single crystal substrate and particularly to amethod for forming an identification mark on a silicon carbide singlecrystal substrate using a laser beam.

BACKGROUND ART

The silicon carbide semiconductor has a larger dielectric breakdownelectric field, a faster saturated drift velocity of electrons, and agreater thermal conductivity than those of the silicon semiconductor.Thus, research and development have been intensively carried out forrealizing a power device which is capable of a large current operationat a high temperature and at a high speed with the use of a siliconcarbide semiconductor as compared with conventional silicon devices.Among others, motors for use in electric motorcycles, electric vehicles,and hybrid vehicles are AC-driven or inverter-controlled, and therefore,development of efficient switching devices for such uses has beenreceiving attention. To realize such power devices, a silicon carbidesingle crystal substrate for epitaxial growth of a high-quality siliconcarbide semiconductor layer is necessary.

Demands for blue laser diodes which are used as a light source forrecording data at a high density and white diodes which are used as alight source in place of a fluorescent lamp or an incandescent bulb havebeen growing. Such light-emitting devices are manufactured using agallium nitride semiconductor, and in some cases, a silicon carbidesingle crystal substrate is used as the substrate for formation of ahigh-quality gallium nitride semiconductor layer. Therefore, there isdemand for a silicon carbide single crystal substrate which is used as asubstrate for manufacture of a semiconductor device for which demand isexpected to undergo a large growth in the future, such as a siliconcarbide semiconductor device, a gallium nitride semiconductor device,etc.

To a semiconductor substrate which is used for manufacture of asemiconductor device, information for identification is provided as anidentification mark for identifying semiconductor substrates andmanaging the process conditions of the manufacture process through whichthey have undergone for each of the semiconductor substrates. Usually,the identification mark has a size which is perceivable by a human eye.However, in other cases, the identification mark is imaged by a cameraor the like and image-processed so as to be detected by a semiconductormanufacturing apparatus or the like.

In forming an identification mark on a semiconductor substrate, a laserbeam is usually used. The semiconductor in a region irradiated with alaser beam is melted and evaporated, whereby a recessed portion isformed in the surface of the semiconductor substrate. The recessedportion constitutes an identification mark. According to the depth ofthis recessed portion, the method for forming an identification mark isgenerally divided into two types. Specifically, formation of anidentification mark with a recessed portion depth of about 0.1 μm to 5μm is referred to as “soft marking”, and formation of an identificationmark with a recessed portion depth of about 5 μm to 100 μm is referredto as “hard marking”. Also, in some cases, the identification mark isconstituted of a recessed portion which is in the form of an independentdot, and in other cases, the identification mark is constituted of oneor more linear grooves.

Silicon carbide is a new semiconductor material and has a higher meltingpoint and a greater hardness than other semiconductor materials whichare widely employed, such as silicon, gallium arsenide, etc. Therefore,it is generally difficult to form a desirable identification mark on asilicon carbide single crystal substrate under the conditions that aresuitable for formation of an identification mark on a silicon substrate.Patent Document 1 discloses the technique of forming an identificationmark which has an excellent visibility, which is realized by irradiatinga silicon carbide single crystal substrate with pulsed laser light whichhas a predetermined pulse shape such that the silicon carbide is melted,whereby a slightly-recessed region is formed which contains a greateramount of carbon or silicon.

CITATION LIST Patent Literature

Patent Document 1: Japanese Laid-Open Patent Publication No. 2006-43717

SUMMARY OF INVENTION Technical Problem

According to the method of Patent Document 1, an identification markwhich is constituted of a slightly-recessed dot is formed. Therefore, itis inferred that, according to the method of Patent Document 1, theidentification mark which is constituted of the dot is formed by softmarking. Formation of the identification mark by soft marking is usuallyperformed on a mirror-finished semiconductor substrate in many cases.However, since formation of the identification mark leads to formationof a bump in the substrate, there is a problem that the flatness of thesubstrate is marred.

Patent Document 1 discloses that a recessed portion in the form of a dotwhich constitutes an identification mark is formed by a region whichcontains a greater amount of carbon or silicon, whereby theperceivability which is attributed to reflected light and transmittedlight is improved. However, there is a problem that an identificationmark which is constituted of a dot is intrinsically inferior invisibility to an identification mark which is constituted of a line.Further, the recessed portion in the form of a dot which constitutes theidentification mark has a small size, and therefore, laser dust, such asa solidified substance of silicon carbide melted by laser irradiation,abrasive grains, or other minute contaminants which can be produced inthe middle of the semiconductor manufacturing process readily remain inthe recessed portion in the form of a dot. Such contaminants remainingin the recessed portion can be a cause for contamination of the surfaceof the substrate when they are separated from the recessed portion, or acause for formation of scars in the surface, in a substratemanufacturing process or a semiconductor device manufacturing processwhich would be performed later.

The present invention solves at least one of the above problems whicharise in the prior art. One of the objects of the present invention isto provide a method for forming a highly-visible identification mark ona silicon carbide single crystal substrate.

Solution to Problem

A method for forming an identification mark on a silicon carbide singlecrystal substrate according to the present invention includes: (a)scanning a principal surface of a silicon carbide single crystalsubstrate with a laser beam at a first energy density such that a grooveis formed in the principal surface of the silicon carbide single crystalsubstrate, thereby forming an identification mark which is constitutedof one or more grooves in the principal surface of the silicon carbidesingle crystal substrate; and (b) scanning an inside of the grooveformed in the principal surface of the silicon carbide single crystalsubstrate with a laser beam at a second energy density that is lowerthan the first energy density.

In a preferred embodiment, a width of the groove is not less than 50 μm,and a depth of the groove is not less than 20 μm.

In a preferred embodiment, at least at a bottom surface of an internalsurface of the groove, the surface roughness Ra is not more than 1 μm.

In a preferred embodiment, the method further includes (c) after step(b), performing mechanical polishing on the principal surface of thesilicon carbide single crystal substrate.

In a preferred embodiment, after step (c), gas phase etching isperformed on the principal surface of the silicon carbide single crystalsubstrate.

In a preferred embodiment, the surface roughness Ra of the principalsurface of the silicon carbide single crystal substrate is not less than0.1 nm and not more than 2.0 nm.

A silicon carbide single crystal substrate of the present invention hasan identification mark on a principal surface of the silicon carbidesingle crystal substrate, the identification mark being constituted ofone or more grooves, wherein a width of the groove is not less than 50μm and less than 0.5 mm, and a depth of the groove is not less than 20μm, and a surface roughness Ra of an internal surface of the groove isnot more than 1 μm.

In a preferred embodiment, a bottom surface of the groove is asolidified surface.

In a preferred embodiment, the bottom surface of the groove has astriped pattern.

Advantageous Effects of Invention

According to the present invention, a silicon carbide single crystalsubstrate can be obtained that has an identification mark on which thereis substantially no contaminant in a groove and which has excellentidentifiability.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram showing a silicon carbide single crystalsubstrate in which an identification mark has been formed by a method ofthe present invention.

FIG. 2 is a flowchart illustrating an embodiment of a method for formingan identification mark on a silicon carbide single crystal substrateaccording to the present invention.

FIG. 3( a) shows a scanning pattern of a laser beam in a rough laserstep. FIG. 3( b) schematically shows a cross section of a groove formedin a principal surface of a silicon carbide single crystal substrate inthe rough laser step.

FIG. 4( a) shows a scanning pattern of a laser beam in a finishing laserstep. FIG. 4( b) schematically shows a cross section of a groove formedin a principal surface of a silicon carbide single crystal substrate inthe finishing laser step.

FIGS. 5( a) and 5(b) are a schematic plan view and a schematiccross-sectional view of a groove formed in a mechanically-polishedprincipal surface of a silicon carbide single crystal substrate.

FIG. 6 is a SEM image of a groove of an identification mark formed bythe method of Example 3.

FIGS. 7( a) and 7(b) are a surface profile along the cross-sectionaldirection and a surface profile along the longitudinal direction of agroove which constitutes an identification mark which is formed by themethod of Example 3.

FIG. 8 is a SEM image of a groove of an identification mark formed bythe method of Comparative Example.

FIGS. 9( a) and 9(b) are a surface profile along the cross-sectionaldirection and a surface profile along the longitudinal direction of agroove which constitutes an identification mark which is formed by themethod of Comparative Example.

DESCRIPTION OF EMBODIMENTS

Hereinafter, an embodiment of a method for forming an identificationmark on a silicon carbide single crystal substrate is described withreference to the drawings. FIG. schematically shows a silicon carbidesingle crystal substrate 10 in which an identification mark 14 is formedby a method for forming an identification mark on a silicon carbidesingle crystal substrate according to the present embodiment. Thesilicon carbide single crystal substrate 10 is made of silicon carbidemonocrystal. The polytype of the silicon carbide monocrystal is notparticularly limited. It may be any polytype of silicon carbidemonocrystal. The size and thickness of the silicon carbide singlecrystal substrate 10 are not particularly limited.

The silicon carbide single crystal substrate 10 has a pair of principalsurfaces 10 a and 10 b. The identification mark 14 is formed in oneprincipal surface 10 a. The plane orientation of the principal surfaces10 a and 10 b is not particularly limited. The crystal axis of thesilicon carbide monocrystal and the normal lines of the principalsurfaces 10 a and 10 b may be identical with each other (so-called “justsubstrate”). Alternatively, the normal lines of the principal surfaces10 a and 10 b may form an angle which is greater than 0° with respect tothe crystal axis of the silicon carbide monocrystal (so-called “offsubstrate”). The principal surface 10 a that has the identification mark14 is the rear surface, while the principal surface 10 b is the frontsurface on which a semiconductor device is to be formed.

The principal surface 10 b of the silicon carbide single crystalsubstrate 10 is preferably a mirror surface. Specifically, the surfaceroughness of the principal surface 10 b, Ra, is preferably not more than2.0 nm. This is because a high-quality silicon carbide layer or galliumnitride layer is epitaxially grown on the principal surface 10 b forfabrication of a semiconductor device. The lower limit of the surfaceroughness Ra of the principal surface 10 b is not particularly limited.However, as the surface roughness Ra decreases, the processing of theprincipal surface 10 b requires a longer time, so that the productivityof the silicon carbide single crystal substrate 10 deteriorates. Thus,from the viewpoint of industrial mass productivity, the surfaceroughness Ra of the principal surface 10 b is preferably not less than0.1 nm.

On the other hand, the principal surface 10 a has a surface roughnesswhich is selected according to its use or specifications required of thesilicon carbide single crystal substrate 10. Specifically, the principalsurface 10 a may be a mirror surface or may be a surface finished bymechanical polishing. When the principal surface 10 a is a mirrorsurface, the surface roughness Ra of the principal surface 10 a is notmore than 2.0 nm. When the principal surface 10 a is a surface finishedby mechanical polishing, the surface roughness Ra of the principalsurface 10 a is not less than 50 nm and not more than 1000 nm.

In the present embodiment, the identification mark 14 is formed in thevicinity of an orientation flat 12 of the silicon carbide single crystalsubstrate 10. However, the position of the identification mark 14 is notparticularly limited. The identification mark 14 may be formed at anyother position over the principal surface 10 a.

The identification mark 14 may be constituted of characters which areused in various languages, such as numerals, alphabets, Katakanacharacters, Hiragana characters, Kanji characters, etc., and symbols.The number of characters is not particularly limited. The identificationmark 14 preferably has a size which is perceivable by a naked eye. Forexample, it is preferred that the size of a single character is 0.8 mmor 1.6 mm. The upper limit of the size of a single character of theidentification mark 14 is not particularly limited. However, when thesize of a single character is excessively large, formation of characterstakes a long time. The groove width is preferably not more than 0.5 mm.

As will be described in detail hereinbelow, the aforementionedalphanumeric characters which constitute the identification mark 14 arenot an identification mark which is constituted of recessed portions inthe form of dots but an identification mark which is constituted oflinear grooves. To secure sufficient visibility for a naked eye, thedepth of the groove is preferably not less than 20 μm, and the width ofthe groove is preferably not less than 50 μm.

Hereinafter, a method for forming an identification mark on a siliconcarbide single crystal substrate according to the present embodiment isdescribed in detail with reference to FIG. 1 and the flowchart shown inFIG. 2.

First, a silicon carbide single crystal substrate 10 is provided (stepS11). As described above, the size, thickness, and polytype of thesilicon carbide single crystal substrate 10 and the directions of thenormal lines of the principal surface 10 a and the principal surface 10b are not particularly limited. The principal surface 10 b of thesilicon carbide single crystal substrate 10 before formation of theidentification mark 14 may have a surface roughness which is obtainedafter being finished by mechanical polishing or may be a mirror surface.

On the other hand, the principal surface 10 a preferably has a surfaceroughness which is obtained after being finished by mechanicalpolishing. This is because, when the surface roughness of the principalsurface 10 a is generally equal to a surface roughness which is obtainedafter being finished by mechanical polishing, the laser beam forformation of the identification mark 14 is prevented from passingthrough the silicon carbide single crystal substrate 10 as compared withthe case where the principal surface 10 a is a mirror surface, so thatenergy can be efficiently supplied to the principal surface 10 a of thesilicon carbide single crystal substrate 10, and the groove of theidentification mark 14 can be formed. Further, the principal surface 10a of the silicon carbide single crystal substrate 10 may be directlyirradiated with a laser beam such that the energy of the laser beam canbe supplied to the principal surface 10 a, without providing an energyabsorbing layer on the principal surface 10 a of the silicon carbidesingle crystal substrate 10 for absorbing the energy of the laser beam.Further, when the principal surface 10 a has a mirror surface beforeformation of the identification mark 14, the principal surface 10 a thatis a mirror surface is mechanically polished after formation of theidentification mark 14 as will be described later, and therefore, theprevious mirror finishing step is of no use. In view of suchcircumstances, specifically, the surface roughness Ra of the principalsurface 10 a is preferably not less than 50 nm and not more than 1000nm. To sufficiently obtain the above-described effects, more preferably,the surface roughness Ra of the principal surface 10 a is not less than100 nm and not more than 500 nm.

Then, the principal surface 10 a of the provided silicon carbide singlecrystal substrate 10 is scanned with a laser beam such that theidentification mark 14 is formed in the principal surface 10 a.Formation of the identification mark is realized by forming theidentification mark 14 which is constituted of one or more grooves bythe rough laser process (step S12) and finishing the inside of thegrooves by the finishing laser process (step S13). First, the roughlaser process (step S12) is described.

As the laser light source which emits a laser beam for formation of theidentification mark 14, a various types of laser light sources for usein laser marking may be used. Here, the laser light source may includenot only a light-emitting light source which emits laser light but alsoan optical system which is used for adapting the beam diameter, a Qswitch which is used for pulse driving of a laser beam, and a wavelengthconverter element which is used for adapting the wavelength of a laserbeam. The laser light source which is used in the present embodiment isconfigured to emit a laser beam at a wavelength which is suitable tomelting and evaporation of the silicon carbide monocrystal.Specifically, the laser light source preferably emits a laser beam at awavelength of not less than 532 nm and not more than 1064 nm. A laserlight source which is configured to emit a laser beam at a wavelengthshorter than 532 nm includes an expensive oscillator and is a large-sizedevice. Therefore, particularly, the cost of forming an identificationmark is likely to increase.

The beam diameter of the laser beam emitted from the laser light sourcedepends on the size of the identification mark 14 which is to be formedand the power of the laser light source. For example, the laser lightsource emits a laser beam which has a beam diameter of, for example, notless than 5 μm and not more than 50 μm. The power of the laser lightsource is, for example, not less than 1.0 W and not more than 2.0 W.Using a laser light source whose power exceeds the upper limit is notpreferred because there is a probability that damage, such as a slip, iscaused in crystal.

The principal surface 10 a of the silicon carbide single crystalsubstrate 10 is scanned with a laser beam using a laser light sourcesuch that the identification mark 14 is formed in the principal surface10 a. The principal surface 10 a is scanned at the first energy densitysuch that a groove is formed in the surface 10 a of the silicon carbidesingle crystal substrate 10. FIG. 3( a) is a plan view schematicallyillustrating scanning with a laser beam. A pulsed laser beam is emittedfrom the laser light source to irradiate the principal surface 10 a withevery single pulse of the laser beam which is represented by a beam spot22 with the beam diameter R1 as shown in FIG. 3( a). To irradiate theprincipal surface 10 a of the silicon carbide single crystal substrate10 with the laser beam at a high energy density, the principal surface10 a is preferably irradiated with the laser beam such that beam spots22 each of which is formed by a single pulse successively overlap. Asthe overlapping area of the beam spots 22 increases, heat can be appliedto the principal surface 10 a at a higher energy density. In thisprocess, a groove 16 which has a cross section such as shown in FIG. 3(b) is formed in the principal surface 10 a of the silicon carbide singlecrystal substrate 10.

To form an identification mark 14 such that it is readily perceivable bya naked eye, the width of the groove 16 which constitutes theidentification mark 14 is preferably not less than 50 μm, and the depthof the groove 16 is preferably not less than 20 μm. Usually, the beamdiameter of the laser beam is about several micrometers, which issmaller than the preferred groove width. Therefore, it is preferred toform a groove which has a wider groove than the beam diameter of thelaser beam by moving the laser beam for scanning in the extendingdirection of the groove 16 while the laser beam is also moved forscanning in a direction which is not parallel to the extending directionof the groove 16. Specifically, it is preferred to perform scanningaccording to a scanning pattern 24 which is zigzagged with respect tothe extending direction of the groove 16. When scanning is performedwith the laser beam with the kerf width W2, the groove 16 with the widthW1 is formed.

By the laser beam irradiation, the silicon carbide monocrystal is meltedto a predetermined depth from the principal surface 10 a of the siliconcarbide single crystal substrate 10, and the melted silicon carbidemonocrystal partially evaporates. Part of the melted silicon carbidewhich has not been evaporated then solidifies. As a result, the groove16 which constitutes the identification mark 14 is formed in theprincipal surface 10 a of the silicon carbide single crystal substrate10.

As shown in FIG. 3( b), the internal surface 16 a of the groove 16formed in the principal surface of the silicon carbide single crystalsubstrate 10 is formed by solidification of the melted silicon carbidemonocrystal. Also, minute solidified substances 18 of the solidifiedsilicon carbide are attached onto the internal surface 16 a. On theprincipal surface 10 a extending outside the groove 16, there are alsosolidified substances or a bump 19 formed by solidification.

The solidified substances 18 produced inside the groove 16 and thesolidified substances or bump 19 produced outside the groove 16 separatefrom the silicon carbide single crystal substrate 10 and attach to theprincipal surface 10 a and the principal surface 10 b of the siliconcarbide single crystal substrate 10 as contaminants so that they cancause adverse effects, become the cause of scratches in the principalsurface 10 a or the principal surface 10 b, or turn to dust to becomethe cause of contamination of other substrates or contamination insidethe semiconductor device, in a subsequent step for fabrication of thesilicon carbide single crystal substrate 10 or a manufacture step formanufacturing a semiconductor device using the completed silicon carbidesingle crystal substrate 10. For example, when an abrasive agent whichis for use in the subsequent step that is for fabrication of the siliconcarbide single crystal substrate 10 is brought into the groove 16, theabrasive agent is trapped by the internal surface 16 a because thesurface roughness of the internal surface 16 a of the groove 16 islarge, so that there is a probability that the abrasive agent cannot beremoved from the groove 16 even by washing.

In the method for forming an identification mark on the silicon carbidesingle crystal substrate 10 according to the present embodiment, thefinishing laser process (step S13) is performed, after the rough laserprocess, on the groove 16 which has been formed by the rough laserprocess in order to solve the above problems. By the finishing laserprocess, the solidified substances 18 and the bump 19 that havepreviously been described are again melted and evaporated such that thesolidified substances 18 and the bump 19 are removed. Further, theinternal surface 16 a is melted and solidified so as to have a smoothinternal surface. The solidified substances 18, the bump 19, and theinternal surface 16 a are formed by solidification of melted siliconcarbide monocrystal, so that they are amorphous or have lowcrystallinity. Further, in the rough laser process, carbon or siliconselectively evaporates, so that solidified substances 18, the bump 19,and the internal surface 16 a have a composition in which silicon orcarbon is excessively contained or a composition in which silicon orcarbon is bound to oxygen. These can be melted and evaporated even whenthe applied energy is not as large as the first energy density in therough laser process.

For example, where the processing energy is estimated by a product ofthe melting point and the thermal conductivity and the processing energyof silicon carbide is 1, the processing energy of Si is about 0.2 andthe processing energy of SiO₂ is about 0.02. Thus, by scanning theinside of the groove 16 which has been formed in the surface 10 a of thesilicon carbide single crystal substrate 10 at the second energy densitythat is lower than the first energy density, the solidified substances18 and the bump 19 can be removed, and also, the internal surface 16 acan be smoothed. Further, since the laser beam irradiation is performedat an energy density which is lower than the first energy density, aportion extending outside the groove 16 which is made of silicon carbidemonocrystal would not be newly melted or evaporated. That is, the laserbeam irradiation is preferably performed at the second energy densitysuch that the silicon carbide monocrystal is not melted or evaporated.Thus, production of new solidified substances 18 or bump 19 by thefinishing laser process is prevented. It is preferred that the ratio ofthe total energy of the finishing laser process to the total energy ofthe rough laser process is about not less than 10% and not more than40%.

A pulsed laser beam is emitted from the laser light source to irradiatethe inside of the groove 16 formed in the principal surface 10 a withevery single pulse of the laser beam which is represented by a beam spot22′ with the beam diameter R1 as shown in FIG. 4( a). In FIG. 4( a), thepositions of the beam spot 22 in the rough laser process are shown bybroken lines. As seen from FIG. 4( a), the overlapping area of the beamspots 22′ is smaller than that of the beam spots 22 so that the secondenergy density is smaller than the first energy density. The othermethods for decreasing the second energy density include decreasing thelaser power and increasing the scanning speed.

Preferably as shown in FIG. 4( a), the scanning direction of the laserbeam in the step of the rough laser process and the scanning directionof the laser beam in the finishing laser process are different from eachother. With this arrangement, the unevenness in the internal surface 16a of the groove 16 which depends on the scanning direction of the laserbeam in the step of the rough laser process is flattened in thefinishing laser process, so that the inside of the groove 16 can befurther smoothed. In the present embodiment, the inside of the groove 16is scanned with the laser beam according to a scanning pattern 24′ whichis parallel to the extending direction of the groove 16. As shown inFIG. 4( a), it is preferred that a portion extending outside the groove16 is also irradiated with the beam spots 22′ in order to remove thebump 19 which has been produced outside the groove 16 of the principalsurface 10 a. With this arrangement, the wall surface of the groove 16is also irradiated with the laser beam at a sufficient energy density,whereby the solidified substances 18 attached to the plane surface areremoved, and the wall surface is smoothed. In other words, it ispreferred that a region which is irradiated with the laser beam by thefinishing laser process entirely includes a region which is irradiatedwith the laser beam in the step of the rough laser process and is alsowider than a region which is irradiated with the laser beam in the roughlaser process. Where a region which is irradiated with the laser beam inthe finishing laser process and a region which is irradiated with thelaser beam in the rough laser process are respectively referred to asthe first region and the second region, the area of the second region ispreferably 100% or more of the area of the first region. To remove thebump 19 which is produced outside the groove 16, the area of the secondregion is preferably 110% or more of the area of the first region.

As shown in FIG. 4( b), by the finishing laser process, the solidifiedsubstances 18 which have been formed inside the groove 16 are removed,and the internal surface 16 b of the groove 16 is further smoothed. Thebump 19 which has been produced outside the groove 16 of the principalsurface 10 a is also removed. As a result of the finishing laserprocess, the surface roughness Ra is not more than 1 μm at least at thebottom surface of the internal surface 16 b of the groove 16.

The finishing laser process may be performed after the entirety of theidentification mark 14 is formed by the rough laser process and therough laser process is completed, or may be performed on parts of theidentification mark 14 which have undergone the rough laser process oneafter another. In this case, the rough laser process and the finishinglaser process are concurrently performed, and it is therefore preferredto provide a laser light source for the rough laser process and anotherlaser light source for the finishing laser process. Where the pulseinterval of the laser beam in the rough laser process is t and theinterval between the rough laser process and the finishing laser processwhich are performed in an identical region of the identification mark 14is T, T is sufficiently longer than t. That is, T>>t, so that part ofthe silicon carbide monocrystal which is melted by the rough laserprocess is solidified and sufficiently cooled before the finishing laserprocess.

Formation of the identification mark 14 in the silicon carbide singlecrystal substrate 10 may be completed through the above-describedprocesses. Alternatively, finishing of the identification mark 14 orfinishing of the principal surface 10 a in which the identification mark14 is provided may be performed. When mechanical polishing is performedon the principal surface 10 a of the silicon carbide single crystalsubstrate 10 in which the identification mark 14 has been formed suchthat the surface roughness of the principal surface 10 a is reduced,when there is a minute bump 19′ remaining outside the groove 16 of theprincipal surface 10 a and the bump 19′ is to be removed, or when thesurface roughness of the internal surface 16 b of the groove 16 isfurther reduced, mechanical polishing is performed on the principalsurface 10 a of the silicon carbide single crystal substrate 10 (stepS14). Specifically, the principal surface 10 a of the silicon carbidesingle crystal substrate 10 is mechanically polished using a metalsurface plate and an abrasive agent. In this process, the abrasive agententers the inside of the groove 16 so that the internal surface 16 b ofthe groove 16 is also polished with the abrasive agent. As a result, asshown in FIGS. 5( a) and 5(b), the silicon carbide single crystalsubstrate 10 is obtained in which the surface roughness of the internalsurface 16 b′ is small and which has a principal surface 10 a′ with areduced surface roughness. Further, the bump 19′ remaining outside thegroove 16 also can be removed by this process.

To remove a damage layer which is formed over the surface of theprincipal surface 10 a of the silicon carbide single crystal substrate10 due to the laser beam irradiation or solidification of melted siliconcarbide, gas phase etching of the principal surface 10 a may beperformed (step S15). Examples of the etching by the gas phase methodwhich can be employed in the present embodiment include ion etching,sputter etching, reactive ion etching, plasma etching, reactive ion beametching, and ion beam etching. Any other gas phase etching method may beemployed.

The type of the gas used in the gas phase etching is not particularlylimited. However, it is preferred to use a gas which contains fluorine,such as carbon tetrafluoride or sulfur hexafluoride or a gas which hasreactivity with silicon carbide, such as hydrogen. Further, oxygen maybe added in order to enhance oxidation. The conditions for the etching,such as the power to apply, depend on the apparatus used for theetching, for example. The etching rate preferably does not exceed 10μm/h. If the etching rate exceeds 10 μm/h, the etching conditions wouldbe excessively intense for the principal surface 10 a of the siliconcarbide single crystal substrate 10 so that, probably, the principalsurface 10 a is damaged by ion collision or the surface morphology ofthe principal surface 10 a after the etching deteriorates. Thus, thisexcessive etching rate is not preferred.

Since the thickness of the damage layer is small, the etching that isbased on the gas phase method does not need to be performed for a longperiod of time. In the gas phase etching, the etching progressesgenerally uniformly so that the surface roughness of the principalsurface 10 a scarcely varies. Thus, a principal surface can be obtainedin which the surface roughness of the principal surface 10 a before thegas phase etching is generally maintained and from which the damagelayer has been removed.

If the principal surface 10 b of the silicon carbide single crystalsubstrate 10 is a surface which is finished by mechanical polishing information of the above-described identification mark, mechanicalpolishing and mirror polishing may be performed on the principal surface10 b after the formation of the identification mark.

By the method for forming an identification mark on a silicon carbidesingle crystal substrate according to the present embodiment, a siliconcarbide single crystal substrate 10 with an identification mark 14 isobtained that is constituted of a groove 16 which has such a depth and awidth that excellent visibility is achieved. Solidified substances 18and the like are scarcely remaining in the groove 16 which constitutesthe identification mark 14, and at least the bottom surface of theinternal surface 16 a has a surface roughness of not more than 1 μm.Therefore, even when mechanical polishing, CMP (chemical mechanicalpolishing), or the like, is further performed on the principal surface10 a in a subsequent step, the solidified substances 18 are preventedfrom separating from the groove and causing scratches in polishing ofthe principal surface 10 a. Also, since the internal surface 16 b of thegroove 16 is smooth, the abrasive agent would not enter or reside in thegroove 16. Thus, an excellent silicon carbide single crystal substrate10 is obtained in which occurrence of various problems which areattributed to formation of the identification mark 14 is prevented inthe process of fabricating the silicon carbide single crystal substrate10 or in the process of manufacturing a semiconductor device.

EXAMPLES

Hereinafter, an example of formation of an identification mark on asilicon carbide single crystal substrate with the use of a method forforming an identification mark on a silicon carbide single crystalsubstrate according to the present embodiment is described.

A 4H silicon carbide single crystal substrate with a diameter of 3inches was provided. The surface roughness Ra of the principal surfacein which an identification mark was to be formed was 0.3 μm. The laserlight source used was a Nd:YAG laser (wavelength: 1064 nm, power: 1.5 W)manufactured by ESI, Inc. This laser light source had a Q-switch and wasused to perform the rough laser process and the finishing laser processunder the conditions of Examples 1, 2, and 3 as shown in Table 1 suchthat an identification mark of nine characters was formed. InComparative Example, only the rough laser process was performed forformation of an identification mark. In Table 1, the kerf width refersto W2 of FIG. 3. When the kerf width was 0, the principal surface of thesubstrate was scanned with a laser beam along a groove to be formed. Forexample, when the width of the groove was about three times the diameterof the laser beam, the principal surface was scanned with the laser beamsuch that spots in the left column shown in FIG. 3 were drawn, theprincipal surface was then scanned with the laser beam such that spotsin the center column were drawn, and lastly, the principal surface wasscanned with the laser beam such that spots in the right column weredrawn. In Table 1, the energy density is a relative value which wasdetermined with respect to the energy density of Comparative Examplewhich was assumed as 100. The total energy refers to a total energywhich was supplied to the substrate by laser beam irradiation formarking a straight line of 1 mm in each of the rough laser process andthe finishing laser process. The energy ratio refers to the ratio of thetotal energy of the finishing laser process to the total energy of therough laser process.

After the formation of the identification mark by the laser, theprincipal surface in which the identification mark was formed wassubjected to mechanical polishing with the use of a diamond slurry inwhich diamond particles with the average particle diameter of 5 μm werecontained as the abrasive agent.

After the mechanical polishing, the inside of the groove constitutingthe identification mark was observed with an optical microscope to checkwhether there was an attached substance, such as a solidified substance,on the bottom surface and the lateral surfaces of the groove. Further,the depth and the width of the groove were measured using an opticallength-measuring microscope. The measurement was performed at anarbitrary position in the groove, where the groove width on thesubstrate surface and the groove depth from the substrate surface weremeasured. The measurement was carried out at one arbitrary position ineach of the grooves of three out of nine characters. Further, thesurface roughness Ra of the bottom surface of the groove was measuredusing the optical interference type surface roughness measuringapparatus HD-2000 manufactured by Veeco Instruments Inc. The measurementwas performed on a central portion at an arbitrary position in thegroove, along the line direction (the longitudinal direction of thegroove), in the length of about 0.2 mm. The measurement was carried outalong one arbitrary line in each of the grooves of three out of ninecharacters. The results are shown in Table 2.

TABLE 1 Exam- Exam- Exam- Compar- ple ple ple ative Ex- 1 2 3 ampleRough Q rate (Hz) 7000 500 500 3000 Laser Power (%) 100 100 100 100Process Speed (mm/s) 40 10 15 8 Kerf Width 0 0.08 0.08 0.15 (mm) EnergyDensity 70 75 72 100 Number of 3 1 1 1 Scanning Cycles Total Energy 7881200 1200 4200 (W) Finish Q rate (Hz) 500 500 500 None Laser Power (%)100 100 100 Process Speed (mm/s) 10 10 10 Kerf Width 0 0 0 (mm) EnergyDensity 10 10 10 Number of 3 3 5 Scanning Cycles (with offset) TotalEnergy 225 225 375 (W) Energy Ratio 29 19 31 (%)

TABLE 2 Exam- Exam- Exam- Compar- ple ple ple ative Ex- Evaluated Items1 2 3 ample Attached Substance Groove No No No Yes Inside Groove BottomWall No Yes No Yes Surface Groove Depth (μm) Before 45 50 50 60 PolishGroove Width (μm) Before 45 100 110 150-170 Polish After 30 80 80 170Polish Surface Roughness of Groove 0.4-0.6 0.4-0.6 0.4-0.6 5.0-7.0Bottom Surface Ra (μm)

As seen from Table 2, no attached substance was found at the bottomsurface of the groove in either of Examples 1, 2, and 3. In Examples 1and 3, no attached substance was also found at the wall surface of thegroove. On the other hand, in Comparative Example, attached substanceswere found at the bottom surface and the wall surface of the groove.This is probably because, in the methods of Examples 1, 2, and 3,solidified substances in the groove were removed by the finishing laserprocess. It was found from the results of Examples 1, 2, and 3 thatattached substances can be entirely removed so long as the energy of thefinishing laser process is approximately not less than 19% and not morethan 31% of that of the rough laser process. It is understood that, whena margin of about 10% is considered, the energy ratio only needs to beapproximately not less than 10% and not more than 40%.

In Example 2, the reason why there was an attached substance on the wallsurface is probably that the laser beam of the finishing laser processfailed to irradiate the wall surface inside the groove with a sufficientintensity. On the other hand, in Example 3, the finishing laser processwas performed through five cycles, and in each scanning cycle, theposition of the beam was offset by 0.02 mm. Therefore, it is inferredthat the beam of the finishing laser process successfully uniformlyirradiated the entire surface inside the groove.

In each of Examples 1, 2, and 3, the surface roughness Ra of the bottomsurface of the groove of the formed identification mark was in the rangeof 0.4 μm to 0.6 μm. On the other hand, in Comparative Example, thesurface roughness Ra of the bottom surface of the groove was in therange of 5.0 μm to 7.0 μm. It was found from this result that thesurface roughness of the internal surface of the groove of theidentification mark which was formed according to the methods ofExamples 1, 2, and 3 was improved to about 1/10 of the surface roughnessof the groove which was formed according to the conventional method. InExamples 1, 2, and 3 and Comparative Example, reduction of the surfaceroughness Ra by the mechanical polishing is estimated at about 50 nm to100 nm, and therefore, the above-described difference in surfaceroughness Ra is not attributed to the mechanical polishing which isperformed after the formation of the identification mark. In Examples 1,2, and 3, it can be said that the surface roughness Ra of the bottomsurface of the internal surface of the groove of the identification markbefore the mechanical polishing is at least not more than 1 μm.

It was confirmed that the identification marks of Examples 1, 2, and 3had improved visibility for a naked eye as compared with ComparativeExample. It was also confirmed that the identification marks which wereformed by the methods of Examples 2 and 3 had further improvedvisibility for a naked eye as compared with the identification markwhich was formed by the method of Example 1.

FIG. 6 is an enlarged SEM image showing a portion of a groove of anidentification mark which was formed by the method of Example 3. As seenfrom FIG. 6, there was substantially no contaminant on the bottomsurface and the lateral surfaces of the groove. Also, there wassubstantially no unevenness in the internal surface of the groove, andit is appreciated that the surface roughness of the internal surface wasvery small. Particularly, it can be seen that the bottom surface of thegroove was a solidified surface, and it had a striped pattern which wasgenerally perpendicular to the extending direction of the groove. Thisis probably because a solidified substance was removed by the finishinglaser process, and as a result, traces were formed at the bottom surfaceof the groove due to sequential melting and solidification of siliconcarbide monocrystal along the traveling direction of the beam spot inthe rough laser process, i.e., the scanning direction of the laser beam.Formation of a solidified surface over the internal surface of thegroove prevented minute contaminants from remaining on the surface.

It is also seen that edges which defined the groove were also sharp, andthe principal surface extending outside the groove was flat. It wasconfirmed that an identification mark having a desired shape was alsoformed by the method of Example 1. In view of these circumstances, weconsider that the identification mark formation methods of Examples 1and 3 are more preferred among Examples 1, 2, and 3.

FIGS. 7( a) and 7(b) respectively show a surface profile along adirection perpendicular to the extending direction of a groove of anidentification mark formed by the method of Example 3 and a surfaceprofile along the extending direction of that groove. As seen from thesegraphs, of the internal surface of the groove, at least the bottomsurface had a surface roughness Ra of not more than 1 μm.

FIG. 8 is an enlarged SEM image showing a portion of a groove of anidentification mark which was formed by the method of ComparativeExample. As seen from FIG. 8, there were a large number of smallsolidified substances attached onto the internal surface of the grooveso that the internal surface of the groove had an uneven shape. Also,the principal surface extending outside the groove was not flat but hadbumps. FIGS. 9( a) and 9(b) respectively show a surface profile along adirection perpendicular to the extending direction of a groove of anidentification mark formed by the method of Comparative Example and asurface profile along the extending direction of that groove. As seenfrom these graphs, the internal surface of the groove had a surfaceroughness Ra of not less than several tens of micrometers, so that theinternal surface of the groove was not smooth.

From the above results, it was found that an identification markconstituted of a groove which has no contaminant attached onto theinternal surface and of which the internal surface is very smooth can beformed by the methods of Examples in which the rough laser process andthe finishing laser process are performed. Employing a mark which is inthe form of a groove rather than dots contributes to excellentidentifiability. It was found that, from the viewpoint of visibility, akerf width is provided, and the scanning pattern of the laser beam iszigzagged in such a manner that a groove width of not less than about 50μm is secured, whereby an identification mark with excellent visibilitycan be formed.

INDUSTRIAL APPLICABILITY

The present invention is suitably applicable to a silicon carbide singlecrystal substrate which is used in various uses, including manufactureof a semiconductor device.

REFERENCE SIGNS LIST

-   10 silicon carbide single crystal substrate-   10 a, 10 b principal surface-   12 orientation flat-   14 identification mark-   16 groove-   18 solidified substance-   19 bump-   22 beam spot-   24 scanning pattern

The invention claimed is:
 1. A method for forming an identification mark on a silicon carbide single crystal substrate, comprising: (a) scanning a principal surface of a silicon carbide single crystal substrate with a laser beam at a first energy density such that a groove is formed in the principal surface of the silicon carbide single crystal substrate, thereby forming an identification mark which is constituted of one or more grooves in the principal surface of the silicon carbide single crystal substrate; and (b) scanning an inside of the groove formed in the principal surface of the silicon carbide single crystal substrate with a laser beam at a second energy density that is lower than the first energy density.
 2. The method of claim 1, wherein a width of the groove is not less than 50 μm, and a depth of the groove is not less than 20 μm.
 3. The method of claim 1 wherein, at least at a bottom surface of an internal surface of the groove, the surface roughness Ra is not more than 1 μm.
 4. The method of claim 1, further comprising (c) after step (b), performing mechanical polishing on the principal surface of the silicon carbide single crystal substrate.
 5. The method of claim 4 wherein, after step (c), gas phase etching is performed on the principal surface of the silicon carbide single crystal substrate.
 6. The method of claim 1, wherein the surface roughness Ra of the principal surface of the silicon carbide single crystal substrate is not less than 0.1 nm and not more than 2.0 nm. 